1. Field of the Invention
The present invention relates generally to electromagnetic interference (EMI) protective measures and, more particularly, EMI protective measures for printed circuit boards.
2. Related Art
Most countries have regulations that limit the amount of electromagnetic emissions that electromagnetic equipment may produce. Electromagnetic emissions are the unwanted byproduct of high-frequency electronic signals necessary, for example, to operate an electronic microprocessor or other logic circuitry. The resulting electromagnetic interference (EMI) is problematic when it interferes with licensed communications such as television, radio, air communications and navigation, safety and emergency radios, etc. This type of interference has historically been known as radio-frequency interference (RFI). See CFR 47 part 15 and ANSI publication C63.4-1992 for regulations in the United States, or CISPR publication 11 or 22 for international regulations. Also, “Noise Reduction Techniques in Electronic Systems” by Henry W. Ott, serves as a comprehensive reference on the current art for the control of EMI, and the broader topic known as electromagnetic compatibility (EMC).
To meet EMI regulations, most electronic equipment currently employs a combination of two approaches commonly referred to as “source suppression” and “containment.” Source suppression attempts to design components and subsystems such that only essential signals are present at signal interconnections, and that all non-essential radio frequency (RF) energy is either not generated or attenuated before it leaves the component subsystem. Containment attempts have traditionally included placing a barrier around the assembled components, subsystems, interconnections, etc., to retain unwanted electromagnetic energy within the boundaries of the product where it is harmlessly dissipated.
This latter approach, containment, is based on a principle first identified by Michael Faraday (1791-1867), that a perfectly conducting box completely enclosing a source of electromagnetic emissions prevents those emissions from leaving the boundaries of the box. This principle is employed in shielded cables as well as in conventional shielded enclosures. Conventional shielded enclosures are typically implemented as a metal box or cabinet that encloses the equipment. The metal box is commonly referred to as a metallic cage and is often supplemented with additional features in an attempt to prevent RF energy from escaping via the power cord and other interconnecting cables. For example, a product enclosure might consist of a plastic structure with a conductive coating on the surface. This approach is commonly implemented in, for example, cell phones. More commonly, the metal enclosure is implemented as a metal cage located inside the product enclosure. Since the EMI suppression necessary for the entire product or system requires that only a portion of the product be shielded, such metallic cages are commonly placed around selected components or subsystems.
There are numerous drawbacks to the use of such metallic cages primarily relating to the lack of shielding effectiveness. Electromagnetic energy often escapes the metallic cage at gaps between the metallic cage and the printed circuit board. Electrical gaskets and spring clips have been developed to minimize such leakage. Unfortunately, such approaches have only limited success at shielding while increasing the cost and complexity of the printed circuit board. In addition, leakage occurs because the cables and wires penetrating the metallic cage are not properly bonded or filtered as they exit the metallic cage. In addition, the metallic cage creates a stagnant buffer of insulating air around the enclosed component or subsystem causing the temperature of the shielded component or subsystem to increase. In such products, the enclosure typically includes cooling apertures and fans to circulate air around the metallic cage to dissipate the heat. Further drawbacks of metallic cages include the added cost and weight to the printed circuit board assembly, as well as the limitations such metallic cages place on the package design.